1. Field of the Invention
The invention relates to a remote locating system for remote locating intermediate devices in an analog or digital transmission line and for remote detecting at least one fault which interrupts the normal data transmission along the line. More specifically, the invention relates to the remote location of pairs of regenerative repeaters which cause an unacceptable operation in an interconnected transmission line between a monitoring station and a monitored station.
2. Description of the Prior Art
Hereinafter, we shall refer to a multiplex data transmission line which transmits digital signals at a predetermined rate. These digital signals are obtained from time-division multiplexing a given number of data digital component signals. The transmission line is bidirectional and comprises an unidirectional forward transmission channel which transmits a data digital signal from the monitoring station to the monitored station and an unidirectional backward transmission channel which transmits another data digital signal from the monitored station to the monitoring station. The forward and backward transmission channels of the line are made up of line sections which are interconnected by pairs of regenerative repeaters, each of which comprises a pair of cables such as coaxial cables. The data transmission in one direction is independent of the condition of the channel in the other direction.
During normal operation, the remote and monitored stations supply the regenerative repeaters on the forward and backward channels, monitor the quality of data digital transmission along the forward and backward channels and perform the analysis of the features of the transmission line. These features are free from noise, which is determined when the error rate is below a given threshold, and continuous routing of the data transmission. As soon as the monitoring station of the monitored station has detected a fault such as an unacceptable feature, the station produces an alarm signal which warns the monitoring station operator than a fault on the data transmission line must be remote-located.
Usually, fault detection is equivalent to remote-locating the pairs of regenerative repeaters in succession, starting from the monitoring station. In this end, the monitoring station comprises supervisory and control equipment which is controlled by the operator and connected to the transmission terminal of the monitoring station in order to supervise the transmission of remote-locating signals and digital testing signals. The supervisory equipment, which monitors and analyses the transmission features of the line, is also used to eliminate local faults in the data transmission line.
In the prior-art methods, a pair of regenerative repeaters is remote located by the supervisory and control equipment in the monitoring station which remote-controls the connection or looping of the output of the regenerative repeater of each pair which is associated with the forward transmission channel to the input of the regenerative repeater of the pair which is associated with the backward transmission channel. Thus, a digital testing signal is transmitted from the monitoring station via a given number of regenerative repeaters which are associated with the forward transmission channel, is routed through the pair of aforementioned regenerative repeaters and is transmitted to the monitoring station by the same given number of regeneration repeaters which are associated with the backward transmission channel. Some features of the received digital testing signal are analysed to show whether all the pairs of regenerative repeaters preceding the looped regenerative repeater pair operate properly. The analysis indicates the faulty regenerative repeater pair and facilitates the return to normal operation and the maintenance of other faulty pairs of regenerative repeaters by measuring the error rates and the frequency shifts.
During maintenance, the digital testing signal may be a pseudo-random signal transmitted from the transmission terminal of the monitoring station. The group propagation time of the testing signal, which is rounded through a looped regenerative repeater pair, is measured so as to indicate the length of the set of line sections which is monitored. In addition, the test information which is transmitted from the terminal of the monitored station to the supervisory and control equipment of the the monitoring station are used to locate a complete breackdown, such as a break in the transmission line or a high error rate.
Remote-locating systems of this kind are disclosed in certain articles of the French periodical "Cables & Transmission", December 1975.
On pages 174 through 183 of the aforementioned periodical, Y. Samoel describes a remote-locating system for remote-locating faults on a multiplex digital transmission line at 2.048 Mbits/s rate. In order to remote locate a pair of regenerative repeaters, this system uses an additional pair of cable sections which are interconnected to six pairs of regenerative repeaters and transmit a suitable direct current, which is cut N times in order to locate and loop the N.sup.th pair of regenerative repeaters. Each transmitted current loops one pair of regenerative repeaters and each current cut-off unloops the previously looped pair of regenerative repeaters. The number of cut-off in the d.c. supply indicates the address of the pair of located regenerative repeaters.
In this system, each pair of monitoring and monitored stations has to be associated with a pair of cable sections which are used only for remote location, and therefore, are under-used.
To avoid this disadvantage, J. P. Prigent, G. Le Fort and D. Betoule describe, on pages 434 through 456 of the aforementioned periodical, a system for remote-locating regenerative repeaters which are interconnected on a multiplex digital transmission line at 51.747 Mbits/s. This system does not use an additional transmission link which transmits the remote locate controlling signals, but only the coaxial cables which transmit, in normal operation, the data digital signals along the forward and backward channels.
To this end, the remote supply current for the forward channel, which travels the center conductor of the corresponding coaxial cable, is reversed at the beginning of the remote-locating operation. As a result of the inversion, all the pairs of regenerative repeaters in the transmission line are looped and the first pair of regenerative repeaters, which is the nearest pair to the monitoring station, is remote-located. Next, a signal at a predetermined low frequency is transmitted in the forward direction on the backward channel and unloops the first pair of regenerative repeaters and simultaneously loops the second pair of regenerative repeaters, which is then located. The process is repeated in succession, so that the number of the low-frequency signal transmissions indicates the address of a determined pair of regenerative repeaters.
In order to restore the normal operating conditions of the transmission line, the remote supply current is inverted a second time. The second inversion, however, must be preceded by the transmission of a signal which controls the inversion of the remote supply current from the monitoring station to the monitored station.
Furthermore, since the remote supply of the regenerative repeater pairs is limited by its range in accordance with the aforementioned remote-location method, order signals are transmitted necessarily before each current inversion, at the beginning and the end of remote location, not only to the monitoring station but also to the intermediate equipment in the transmission line which performs the remote supply of a regenerative repeater pair group. This intermediate equipment is needed for providing the d.c. supply which is attenuated during transmission along the transmission line. Consequently, in this remote locating system, the range of the remote locating signals is identical with the remote supply range, which means that pairs of regenerative repeaters cannot be remote located if the transmission line is more than 100 km approximatively.